Rafters float the Merced River in Yosemite National Park, which is struggling to accommodate visitors while protecting natural resources. (Getty Images/David McNew)

The National Park System is threatened by pollution, invasive species, climate change and encroaching development. Moreover, budget constraints are making the national parks and other units in the system not only more dangerous to visitors but also less satisfying and less educational, some observers warn. To compensate for funding shortfalls, parks are raising entrance fees, soliciting corporate donations and cutting ranger programs. Underlying the challenges is a fundamental struggle between recreational users — such as snowmobilers and jet skiers — and traditionalists who say preserving the parks' tranquility and fragile resources should always remain the paramount mission. Meanwhile, “gateway communities” say park officials should take their economic survival into account as they try to keep the parks meaningful for 274 million visitors a year and preserve natural values for future generations.